Investigate random failures of unit tests due to matplotlib

[Carsopre]

Every now and then TeamCity fails to fully run all tests due to a matplotlib malfunction (it does not happen locally).
For example: https://dpcbuild.deltares.nl/buildConfiguration/Koswat_ContinuousIntegrationBuild_RunFastTests/2864399?hideProblemsFromDependencies=false&hideTestsFromDependencies=false&pluginCoverage=true&expandBuildProblemsSection=true&expandBuildTestsSection=true&expandBuildChangesSection=true

Error:

>       self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)
E       _tkinter.TclError: Can't find a usable tk.tcl in the following directories:

To do:
Investigate whether we need to adapt the code / tests to avoid this issue or potentially move the tests into a different category (or perhaps rename slow to acceptance and move also these tests there.